1. Field of the Invention
This invention relates to a highly corrosion-resistant surface-treated steel sheet having an unconventional appearance design and excellent functions such as press-processability, electrodeposition coatability and weldability, which is manufactured by forming a thin, uniformly colored coating film on surface(s) of a steel sheet plated with Zn, a steel sheet plated with a zinc-based alloy such as Zn-Cr, Zn-Ni, Zn-Fe, Zn-Al, Zn-Mg or the like, a steel sheet plated with a zinc-based composite containing a metal oxide such as SiO.sub.2, TiO.sub.2, ZrO.sub.2, or the like, a steel sheet plated with Al, a steel sheet plated with alloyed zinc, or a steel sheet having a multilayer of these platings. Such a surface-treated steel has great usefulness in fields requiring its low cost and multifunctions.
2. Description of Related Art
Colored steel sheets have been employed to comply with users' need for omission of coating and high resistance to corrosion. And, recently, there, has also been an increasing need for the coloring of plated steel sheets. However, it has not been possible to use any conventional colored steel sheet in a manner in which plated steel sheets are used, since they usually have a film thickness of 5 .mu.m or more and therefore have insufficient weldability and press-processability.
That is, Japanese Patent KOKOKU (Post-Exam. Publns.) Nos. Sho 45-24230, 47-6882, etc., propose zinc-rich coating compositions, which permit electrodeposition coating. However, the press-processability of these coating compositions is poor and their corrosion resistance and weldability are also insufficient. Japanese Patent KOKOKU (Post-Exam. Publn.) No. Sho 52-44569, Japanese Patent KOKAI (Laid-Open) No. Sho 58-138758, Japanese Patent KOKOKU (Post-Exam. Publn.) Nos. Sho 54-9616 and 58-19706, etc., propose coating compositions incorporating an electrically conductive pigment, which remarkably improve the corrosion resistance of a zinc-plated steel sheet, etc., and also improve the weldability. Since, however, these coating compositions contain a comparatively large-sized pigment such as a zinc powder, metal powder, metal carbide, metal phosphide, or the like, films formed by electrodeposition coating of these coating compositions have a very rough, uneven surface texture and lack coating smoothness. And, since these coating compositions do not have any sufficient press-processability, either, the portions where these coating compositions can be applied are limited.
Further, Japanese Patent KOKOKU (Post-Exam. Publn.) No. Sho 60-33192, Japanese Patent KOKAI (Laid-Open) Nos. Sho 58-224174 and 60-174879, etc., propose organic composite silicate films and steel sheets coated with such a film, in which electrodeposition coating can be carried out to form thin films containing no electrically conductive pigment and having a thickness of 0.3 to 3 .mu.m and steel sheets coated with these films. Although these organic composite silicate films and coated steel sheets have improved high corrosion resistance, weldability, press-processability, etc., they still have a problem in that an undercoat is seen through the films since the films are clear, or a handling problem in that the distinction between a coated surface and an uncoated surface is difficult when one surface of a steel sheet is coated. Furthermore, Japanese Patent KOKOKU (Post-Exam. Publn.) No. 55-5422, Japanese Patent KOKAI (Laid-Open) No. 58-61291, etc., describe colored precoat steel sheets of which the appearance design is improved by forming colored thick coating films having a thickness of 5 to 20 .mu.m. Since, however, the films are thick and soft, these steel sheets have a defect in weldability and are liable to suffer scars in handling.
As described above, in fact, none of the conventionally proposed techniques have met the qualities presently required of a steel sheet coated with a colored thin film.